1. Field of the Invention
This invention relates generally to the field of computer networks and more particularly to the field of CSMA local area networks.
2. Background of the Invention
A wide variety of multiple access schemes (or protocols) has been proposed and established for local area networks over the past several years. These schemes provide a set of efficient rules for providing virtual communication between all points (elements) of the network, while physical communication is provided by the channel itself. All of these protocols attempt to make optimal use of the bandwidth-limited resource, namely the common channel, while simultaneously minimizing the delays experienced by the stations connected to that channel. The available protocols can be broadly classified in three categories, namely, contention, limited contention, and reservation. Notable examples are the CSMA-CD (Carrier Sense Multiple Access With Collision Detection) contention protocol, and the MSAP (or Chlamtac's version, BRAM) reservation protocols.
In CSMA-CD, the time axis is divided into contention and transmission slots, with idle periods occuring in between, when no one is using the channel. The contention slot (d) is the channel (cable) round trip propagation delay, and the transmission slot (T) is the time required to transmit a fixed length packet.
CSMA-CD is a contention protocol developed for Ethernet in which ready stations listen to the channel and transmit only if the channel is found idle. All transmitting stations monitor the channel and terminate their transmission immediately if a collision is detected within a contention slot. The CSMA-CD mechanism is illustrated in FIG. 1.a for the case where there is at least one busy station at all times (no idle periods). An Ethernet coxial cable segment connecting N stations (S.sub.1, S.sub.2, . . . , S.sub.n) via transceivers is shown in FIG. 1.b.
The following is a brief analysis of the efficiency of CSMA-CD for constant loads (say (k) out of a total of N stations are always ready to transmit). In order to find the average length of the contention interval, assume that each ready station transmits (or retransmits after collision) in a contention slot with probability a. The probability A that one station will acquire the channel successfully is given by: EQU A=ka(1-a).sup.k-1 ( 1)
This quantity, A, is maximized for a=1/k.
The probability, P.sub.j that a contention interval is of length j, is given by: EQU P.sub.j =A(1-A).sup.j-1 ( 2)
Therefore, the average length of the contention interval, j, is: ##EQU1##
Noting that for each packet transmission, j contention slots on the average are lost, channel efficiency may be expressed as: ##EQU2##
It is well known that as the number of stations N increases, the average length of contention increases. In high loading environments, this can result in unacceptable delays. The present invention relives this problem allowing small delays for heavily loaded systems while providing little impact on lightly loaded systems.